1. Field of the Invention
The present invention relates to an X-ray computerized tomography apparatus which compensates for variations in sensitivity among channels.
2. Description of the Related Art
An X-ray computerized tomography apparatus (hereinafter abbreviated to an X-ray CT apparatus) directs X-rays to a subject under examination from a plurality of different directions to thereby acquire a plurality of pieces of projection data in different transmission paths, performs reconstruction processing on those projection data to thereby compute a CT value corresponding to the X-ray absorption of each portion in a plane section of the subject, and produce a reconstructed image (tomogram) of the plane section by forming these CT values into a two-dimensional array.
FIG. 1 schematically illustrates the internal structure of a gantry that is a main component of an X rays CT apparatus of the third generation. X-rays emitted from an X-ray tube 1 are directed onto a subject under examination P through a slit 3. A multichannel X-ray detector 2 is placed so that it is opposed to the X-ray tube 1 with the subject P interposed therebetween. Rotating mechanisms not shown cause the X-ray tube 1 and the X-ray detector 2 to rotate around the subject P. X-rays emitted from the X-ray tube 1 are partly absorbed by tissues of the subject P and the absorption of the X-rays is detected as projection data by the X-ray detector 2. The emission and detection of X-rays are repeated each time the X-ray tube and the detector are rotated through a small angle. Thereby, projection data are detected from many directions. These projection data are fed into an image reconstructing unit via a data collector. The image reconstructing unit performs reconstruction processing on the projection data from various directions to obtain CT values of various portions of a plane section of the subject. These CT values, combined with position data, are sent to an image display device or image storage device.
FIG. 2 is an enlarged view of the A portion of the X-ray detector of FIG. 1. The X-ray detector 2, when it is of a solid-state type, consists of an array of a plurality of X-ray detector elements 4 which are each composed of a scintillator and a photodiode and which are arranged along circumference with the center at the X-ray focal point. The detector elements 4 are separated by collimators 5. FIG. 3 is a view of the X-ray detector 2 seen from the X-ray tube 1. Manufacturing errors of processing and assembly and nonuniformity of materials used will produce variations in sensitivity among X-ray detector elements (channels). In general, these variations are compensated for by correcting output values of the X-ray detector elements obtained when a subject under examination is actually imaged with output values of the detector elements obtained when a water phantom is imaged, i.e., calibration data.
Nowadays the most commonly used type of an X-ray tube is a rotation anode X-ray tube. In this type of X-ray tube, X rays are produced when a rotation target electrode (anode), which is made of tungsten or the like, is bombarded with a beam of electrons emitted from a cathode. This point of bombardment is the focal point of X rays. When the target is made of tungsten, the efficiency of energy conversion from electron beam to X rays is less than 1%, and most of the remainder of the energy is converted to heat energy. The rotation anode X-ray tube was developed for higher heat resistance. Part of the heat generated in the target is radiated through a shaft that mounts the target rotatably.
The repetition of X-ray emission increases stored quantity of heat in the target and the temperature of the shaft increases correspondingly. As a result, the shaft expands and the X-ray focal point shifts in the direction of slice accordingly. It is common that the shaft is provided in parallel with the direction of slice. As was mentioned in connection with FIG. 3, the manufacturing errors of processing and assembly and the nonuniformity of materials produce variations in sensitivity among channels, which will change with the shift of the X-ray focal point. High-sensitivity solid-state X-ray detectors that are the most used type nowadays will follow changes of the variations faithfully.
The above-mentioned calibration data are those obtained under a fixed focal point. Thus, the conventional correcting method cannot compensate for changes of variations in sensitivity among channels due to the shift of the X-ray focal point, which will produce a ring-like artifact on a tomography image.
To remedy such a problem, a method is disclosed in U.S. Pat. No. 991,189 which moves the slit according to the shift of the X-ray focal point. However, this method is not preferable because the slice plane is caused to shift.